A semiconductor package, such as a BGA, includes an array of multiple projecting ball electrodes. In order to mount a semiconductor package on the obverse face of a printed wiring board, multiple methods are conventionally utilized such as: a disc squeegee method, a paste screen print method, or a combination of the two. These methods generally apply a solder paste to either the ball electrodes of the semiconductor package or to the wiring pattern of the printed wiring board.
The disc squeegee method is a technique whereby a disc coated with a solder paste is rotated and the solder paste is smoothed using a squeegee. Thereafter the solder paste is applied to ball electrodes by dipping them in the solder paste. However, with this method, it is difficult to adjust or maintain the rotatory flatness of the rotary disc, and since the rotatory flatness varies each time the disc is detached, uniform volume of the transferred solder paste is not available for all the ball electrodes. This problem is especially seen in warped semiconductor packages, resulting in non uniform ball electrode heights leading to coating discrepancies occurring during the spreading of the solder paste. Another problem occurs when the ball electrodes are dipped to half their height leading to bridges forming between the electrodes.
The paste screen printing method is a technique whereby screen printing is used to apply solder paste to the wiring pattern of a printed wiring board before a semiconductor package is mounted. However, this method is difficult to perform where a wiring pattern has a pitch of 0.3 mm or smaller. Further, since this method does not allow for the coating of too much solder paste, one is unable to absorb the non uniform height of a warped semiconductor package with excess solder paste. In addition, it is difficult to utilize this method with a module where an SMC (Surface Mount Component) is mounted, especially a PoP module that requires the application of a solder paste to a wiring pattern on a component mounting side.
By using a combined method in which both the disc squeegee and the paste screen printing method are used, the amount of solder paste applied can be increased, however a increase in costs can not be avoided.
In accordance with a reduction in the size and thickness of a semiconductor package, the pitch of the ball electrodes is reduced, and the diameter of the ball electrodes becomes minute (i.e. about 0.4 to 0.2 mm). Therefore, the amount of solder paste needed for stable application is decreased.
Further, if a manufacturing variance of ±0.1 mm is present in the heights of ball electrodes, due to the warping of a semiconductor package, a difference of ±0.1 mm will be present between the center and the peripheral portion. Thus, when too little solder paste is applied, some of the ball electrodes will not be coated with solder paste. On the other hand, when too much solder paste is applied, either bridges will occur between the ball electrodes, or the solder paste will touch the obverse face of the board. Therefore, when a conventional method is used to apply solder paste, the soldered ball electrodes must be inspected, either visually or using X-ray.